1. Field of the Invention
The present invention relates to a bed system for radiation therapy including a bed for holding a patient stationary when treatment is performed by applying radiation irradiated from an irradiating section to a diseased part, and more particularly to a bed system for radiation therapy which is suitable for use in a proton radiation therapy device having a rotation irradiation chamber (referred to as a gantry) and can achieve irradiation from any direction and distance, especially non-complanar irradiation whose irradiation direction is not perpendicular to the patient axis, to a patient held. stationary on a bed.
2. Description of the Prior Art
Conventional cancer therapy based on radiation of active rays uses X-rays, gamma rays, electron beams, fast neutron beams, etc. These active rays, as shown in FIG. 14, become the strongest at sites close to the surface of a patient, and thus may inflict damages on normal tissues close to the body surface when those rays are directed towards a cancer in a deeper part of the body. By the way, a proton or a particle which comes into being when a hydrogen atom has been removed of the electron, has a positive charge, and has a mass 1836 times as large as that of electron, can be accelerated under a high energy state by an accelerator to give a proton beam. The proton beam is characterized by having the maximum dose peak or a Bragg peak P at a certain depth from the body surface, and then declining rapidly to zero.
This is because, as the electric force a proton exerts on electrons becomes large in proportion to its proximity to the latter, when the proton has a high kinetic energy and runs at a high speed, the time for the proton to interact with nearby electrons is short, and ionization is small in magnitude, but, when it loses the kinetic energy to nearly make a stop, the time for interaction becomes long and ionization rapidly increases in magnitude.
Thanks to this nature peculiar to protons, it is possible to apply proton beams for cancer therapy keeping normal cells other than a cancer comparatively free from damages, even if the cancer lies in a deeper part of the body. Further, as the radiation-based biological effect (RBE) of a proton beam is nearly equal to that of X-rays, the proton radiation therapy is advantageous in that it can make the most of knowledge and experience accumulated in the field of conventional X-ray radiation therapy. With these features, the proton radiation therapy device is being introduced as a therapy means to treat a cancer without removing any functional organs and encroaching on the quality of life.
Such a proton radiation therapy device is generally constituted of a therapy device A, an attachment B, and an attendant facility device C, as shown in FIG. 15.
The therapy device A consists of, for example, a proton beam accelerator 1 which accelerates protons, changes the energy of the proton beam taken out, and limits the energy from being expanded, a beam transport system (BTS) 2 which secures a stable orbit for the proton beam to transport it to an irradiation chamber without loss, and a rotation irradiation device (gantry) 3 and a stationary irradiation device 4 which form and process the proton beam to irradiate it to a lesion position of a body exactly.
The proton beam accelerator 1 is composed of, for example, a cyclotron which is the main body of an accelerator and accelerates protons to an energy of 235 MeV and an energy selection system (ESS)which changes the energy of the proton beam irradiated from the cyclotron, if desired, while limiting the energy dispersion.
The rotation irradiation device 3 is composed of an irradiating section (nozzle) which realizes irradiation requirements such as an irradiation field and irradiation depth, a terminal section of the beam transport system (BTS) 2 which transports the beam to an entrance of the irradiation section, and a structure in which the nozzle and the terminal section of the beam transport system 2 are installed and which irradiates the beam in any direction from the nozzle attached to its tip, and a bed system including a positioning device for diseased part of a patient is located adjacent to the rotation irradiation device 3.
The stationary irradiation device 4 is substantially identical to the rotation irradiation device 3, but it is different from the rotation irradiation device 3 that proton beam irradiated from the nozzle are fixed only, for example, in the horizontal direction.
The attachment B comprises a diagnostic device, a therapy planning system for planning a irradiation therapy, and a therapy implement machine tool. The diagnostic device consists of an MRI and CT scanner for acquiring diseased part information in a body of a patient and an X-ray simulator for confirming the positioning of the diseased part in the body. The therapy planning system is composed of hardware and software for achieving the irradiation therapy planning according to the diseased part information in the body obtained from the diagnostic devices. The therapy implement machine tool is constituted of an NC electric discharge machine, NC machining center, and NC three-dimensional coordinate measuring instrument which process a patient collimator and bolus with on-line according to the output from the therapy planning system. Incidentally, the attachment B goes out the subject matter of the present invention and will not be discussed any further.
The attendant facility device C is composed of a various power sources in which the main components are DC current sources supplying power to the accelerator and the beam transport system and a pure water cooling and supplying facility for directly cooling an electric current conductor (coil). Incidentally, the attendant facility device C also goes out the subject matter of the present invention and will not be discussed any further.
The proton beam therapy device makes the safety to the patient and medical staff to the top priority from a viewpoint that it is a medical device, but on the other hand its safety, operability and easy maintenance are pursued as it is operated with a few people under the initiative of the medical staff. The system adopts a cyclotron as an accelerator and when compared with other types of accelerators the beam generated from the cyclotron has the following property:
(1) Large maximum electric current can be obtained (maximum 300 nA), PA0 (2) The short time fluctuations of its electric current value and the beam shape are very small, PA0 (3) The time fluctuation of the irradiation position of a beam is very small, and PA0 (4) Various structures from continuous beam to pulse beam can be formed in time structure. PA0 (1) The accelerator itself can perform the irradiation for therapy in a temporally and spatially stable condition, so that the system after the accelerator is made to be simplified and reliable (for example, if an irradiation field is less than 20 cm in diameter, a structurally simple and stable dispersion method can be employed), PA0 (2) An appropriate irradiation can be performed without restraining a patient for a long time according to the position of a diseased part fluctuating with breath of the patient regularly and irregularly, PA0 (3) It is provided with a capability sufficiently corresponding to a three-dimensional irradiation which will become an ideal irradiation form in the near future as a therapy irradiation, PA0 (4) The starting-up and shut-down time of the irradiation is short, a lot of time available for therapy can be produced, the operation is simple, and operators with knowledge and experience of an accelerator are not needed, and PA0 (5) The countermeasure against noise resulting from magnetic and high frequency fluctuation to medical electronic devices can be performed easily.
Furthermore, the cyclotron has the following referred characteristics other than the beam properties, that is, the cyclotron adopts a simple configuration in which only three devices are adjustment object devices in regular operation of the accelerator and also it is less prone to affect the performance of an MRI and CT simulator which are susceptible to the rapid fluctuation of magnetic field as the cyclotron is provided with a constant magnetic field unlike other accelerators which produce fluctuation of magnetic field and high frequency positively. These characteristics of the cyclotron results in the following features to the proton beam therapy device:
From the viewpoint of the whole proton beam therapy device, devices around an irradiation therapy section to which patients and medical staffs have to gain access daily are more important than the accelerator judging from securing safety and exerting irradiation and operation performance. The configuration around the irradiation therapy section is composed of, as described above, the irradiation device and the positioning device of a patient, and it is necessary to prioritize securing safety especially for these devices.
As for safety, basically to make the idea of fail-safe thorough, it is incontestable not only to implement the safety policy to device itself, such as mechanical safety design for an electrical machine and selection of materials for preventing radiation deterioration, but also to embody measures for patients and medical staffs, assuming various cases. For example, only to secure safety to the patient, it is necessary to assume various accidents and to embody safety measures, such as prevention of excessive dose irradiation exceeding a predetermined dose, prevention of mechanical trouble accidents caused by gantry structure drive, picture tube drive, and bed drive, securing safety emergency evacuation of patient when a device accident occurs, obviating the urgent fall when a collimator for patient and a bolus are exchanged, and accident detection of a patient and safety urgent countermeasure when irradiation is performed.
The function required for around the irradiation therapy section is to irradiate diseased parts as an irradiation condition created using the therapy planning system, that is, to irradiate proton beam within an allowable error so that the dose distribution and dose value to the diseased part of an irradiation object as planned may be obtained. In order to achieve the irradiation, it is required that the irradiation position of patient's diseased parts to a beam must be determined with sufficient accuracy and the dose distribution planned must be realized accurately using various instruments for beam formation arranged in the nozzle.
In order to satisfy the former requirement, the positioning of a patient's diseased part is performed with a procedure to carry out a precise positioning such that a beam axis and an irradiation center of the diseased part are, at first, made coincide with a criterion marking on a body surface of the diseased part using cross laser pointers arranged in the nozzle and an irradiation space to perform a coarse positioning in the horizontal and vertical directions, and subsequently, a precise positioning is performed by moving a bed so that the X-ray image information in the horizontal and vertical directions of the patient's diseased part obtained from a DRR (Digital Radiography Reconstruction) device disposed in an in radiation space and performing image reconstruction due to electronic signals is brought into agreement with the irradiation position set up in the therapy planning. In addition, as a prerequisite for the precise positioning, it is required that the positional accuracy including the reproducibility of the beam axis (nozzle) and the irradiation center position should be secured sufficiently.
Most of the requirement for the dose distribution of the latter may be solved basically if the beam property including the reproducibility is sufficiently stable temporally and spatially within the representative therapy time, and the latter part is dependent on how the measurement of dose distribution prior to the irradiation therapy using a phantom comprising water or the like derivative the absorption of a human body can be executed precisely and in a short time.
In the radiation therapy of cancer, it is ideal to concentrate a lethal dose of active rays onto the cancer alone without inflicting any irreversible damages to nearby normal tissues. The Proton radiation therapy, as shown in FIG. 14, exploits the feature characteristic with protons that a proton beam incident on a substance gives the maximum dose or Bragg peak P just before it ceases to move. Namely the therapy in question aims at achieving this ideal by covering only the cancerous lesion with that Bragg peak.
By the way, protons obtained from an accelerator are in the form of a slender beam, and its energy is constant (the depth of Bragg peak is also constant). On the other hand, cancerous lesions are varied in size and have complex shapes, and their depths in the body are not constant. Further, the density of tissues through which a proton beam must pass is not constant neither. Accordingly, to achieve an effective radiation therapy, it is necessary to (1) enlarge the proton beam to have a sufficient width to cover the whole cancer lesion in one radiation; (2) adjust the beam energy according to the depth of lesion; (3) give a sufficient energy distribution in depth so that the whole cancer lesion having a certain depth can receive a uniform irradiation; and (4) make corrections according to the irregularities in contour of the lesion, and in density of the tissues through which the proton beam must pass.
It is necessary to irradiate the proton beam adjusted according to the shape and depth of cancer to the cancer tissues inside of a patient correctly as the irradiation condition so that the expected dose distribution and dose value can be achieved within the allowable error.
In order to achieve such irradiation, it is necessary to decide the irradiation position of the patient to the beam precisely as well as to realize the dose distribution planned precisely using such a irradiation field forming device as a bolus or collimator.
In a proton beam therapy device described above, proton beam with high quality is produced from the cyclotron as an accelerator and also the position accuracy including the reproducibility of the positions of the beam axis (nozzle) and the irradiation center can be secured sufficiently in the proton beam irradiated from the nozzle in the direction of the patient, so that a bed used as a treatment table which moves the diseased part of the patient to be positioned has to be provided with a positioning drive means which allows the diseased part to be positioned by moving a human body having a weight of several tens of kg and being like a soft water bag as compared with solid material like a stone to the position in which the proton beam emitted from the nozzle exerts the maximum efficiency quickly and exactly with the minimum delay depending to an inertial force. Furthermore, in the case of disaster, such as earthquake or the like, the radiation of the proton beam must be stopped promptly and also the bed with a patient must be fixed at a predetermined position.
However, a conventional bed used in radiation therapy has only functions such that the bed with a patient held stationary can be inserted into an irradiation chamber in the direction of one axis and the irradiating section can be rotated around the axial center of the patient, so that the bed was not able to realize an irradiation from arbitrary directions and distances required in the radiation therapy, especially, a non-complanar irradiation in which the irradiation direction was not perpendicular to the axial center of the patient.